Human leukocytes express the CD11/CD18 family of receptors, LFA-1 CR3 and p150, 95 that mediates transient adhesion to ligand-coated cells, substrates, or particles. CD11b/CD18 (CR3) on polymorphonuclear leukocytes (PMN) exhibits ligand-independent aggregation in the plane of the membrane, and this reversible aggregation regulates its ability to bind C3bi-coated erythrocytes (Detmers et al., 1987, JCB, 105:1137). The relationship between receptor aggregation and function will be explored for both CD11b/CD18 and other receptors on leukocytes which are structurally or functionally, related to it. Enhancement of the binding activity of CD11b/CD18 by two physiological agents, tumor necrosis factor-alpha and bacterial lipopolysaccharide, will be characterized in detail and compared with the effects of these agents on receptor aggregation. On macrophages the binding and phagocytosis-promoting activity of CD11b/CD18 can be independently modulated by interferon gamma and fibronectin, and the effects of these treatments on the aggregation state of CD11b/CD18 will be examined. To determine whether the exteriorization of intracellular pools of receptor is required to enhance binding or phagocytic activity, the activity of receptors on cytoplasts of PMN will be examined. The role of receptor aggregation will be examined in regulation of another phagocytic receptor that is structurally distinct from CD11b/CD18. Preliminary observations indicate that Fc-mediated phagocytosis is strongly enhanced by LPS. This regulated behavior will be characterized, and immunoelectron microscopy will be used to determine whether aggregation of FcR is responsible. Two lines of experiments will seek to determine the mechanisms underlying the formation of receptor clusters and the consequent regulation of ligand binding. Evidence for physical connections between CD11b/CD18 and the cytoskeleton will be sought in double labelling experiments with antibodies against known cytoskeletal components. In addition, preliminary experiments indicate that CD11b/CD18 is phosphorylated under conditions which activate the binding activity of the receptor. The specificity, time course, and location of these phosphorylation events will be characterized in detail. The CD11/CD18 family is part of a large class of adhesion-promoting receptors termed integrins, and the results of these experiments will be of general relevance to a large number of cell adhesion interactions.